Mercury is a highly toxic and widespread pollutant in the environment. Mercury can be a source of environmental contamination when present in by-products of burning coal, mine tailings and wastes from chlorine-alkali industries.[1,2] These contaminations can cause a number of severe health effects such as brain damage, kidney failure, and various cognitive and motion disorders.[3] Therefore, there is high demand for sensitive and selective mercury detection.
Towards this goal, many mercury sensors based on small fluorescent organic molecules,[4-11] conjugated polymers,[12] foldamers,[13,14] genetically engineered cells,[15] proteins,[16-18] oligonucleotides,[19,20] membranes,[21,22]electrodes,[23,24] and nanomaterials[25-30] have been reported. Despite this progress, few sensors show enough sensitivity and selectivity for detection of mercury in aqueous solutions.
Sensors that meet such requirements remain complicated to design and operate, or are vulnerable to interference, making difficult facile on-site and real-time detection and quantification of mercury. A particular interesting example is environmental-monitoring applications, such as mercury detection in drinking water, in which a detection limit below 10 nM (the maximum contamination level, as defined by the U.S. Environmental Protection Agency (EPA)) is required. However, only a few reported sensors can reach this sensitivity.[11,15,18,26,27] Therefore, a simple sensor with high sensitivity and selectivity for facile on-site and real-time mercury detection is still needed.
Polynucleotides provide an attractive methodology for mercury sensing. Ono and co-workers reported that mercury ion Hg2+ has the unique property of binding specifically to two thymine bases and stabilize thymine-thymine mismatches in a DNA duplex; they demonstrated a fluorescent sensor for Hg2+ ion detection based on this property.[19,31] In their sensor design, one single-stranded thymine-rich polynucleotide was labeled with a fluorophore and quencher at each end. In the presence of Hg2+ ions, the two ends of the polynucleotide became closer to each other through thymine-Hg-thymine base pair formation, resulting in fluorescence decrease due to an enhanced quenching effect between the fluorophore and quencher. A detection limit of 40 nM was reported.
The Hg2+ ion-induced stabilization effect on thymine-thymine mismatches has also been used to design colorimetric sensors with DNA and gold nanoparticles based on labeled[25,29] or label free methods.[27,28,30] Recently, Liu et al. reported a highly sensitive mercury sensor based on a uranium-specific DNAzyme by introducing thymine-thymine mismatches in the stem region of the original DNAzyme.[32] Hg2+ enhanced the DNAzyme activity through allosteric interactions, and a detection limit as low as 2.4 nM was achieved.
Although highly sensitive and selective, this sensor however requires the use of 1 μM UO22+ for DNAzyme activity. This drawback creates the motivation to find an alternative method for mercury sensing, with comparable sensitivity but without the need to use other toxic metal ions as co-factors.
Fluorescent sensors based on structural switching aptamers have been developed to detect a number of non-metal ions such as adenosine-5′-triphosphate (ATP),[33-35] cocaine,[36] thrombin,[37] and platelet-derived growth factor (PDGF).[38] Aptamers switch structure in the presence of an effector usually due to the formation of non-covalent interactions, such as hydrogen bonds, ionic bonds and Van der Waals interactions, between the aptamer binding site and the effector.